Patent GB-646478-A (GB)
Title: Improvements in or relating to speed control apparatus
IPC: G05D13/00
Filed: 1948-02-20
Abstract: 646,478. Automatic speed control systems. AMERICAN BRAKE SHOE CO. Feb. 20, 1948, No. 5056. Convention date, Feb. 21, 1945. [Class 38 (iv)] [Also in Group XXXVII] An automatic control system for maintaining constant the relative speed of two independently driven shafts by control of the speed of the secondary shaft, comprises means for modifying the control action if the absolute speed of the secondary shaft is outside a pretermined range, or for disabling the control system if the speed of the master shaft is outside a predetermined range. The invention is especially applicable to the control of aircraft engines. The Figure shows a system in which the speed of shafts 20-1 and 20-2 are controlled to maintain a constant speed relative to the master shaft 20M. Each shaft is associated with a commutator switch S1, S2 and MS respectively, the commutator segments of each of the switches S1 and S2 being connected to corresponding segments on master switch MS. The negative pole of a source of potential 85 is connected via a slip ring and centrifugal switch elements 71A and 70A to a brush 40 on the master switch MS. Switch S1 has three brushes, 41, 42 and 43. Brush 41 is connected via a slip-ring to a master relay 115 and thence via contacts 121, 122 to the positive pole of source 85. Brush 42 is connected via centrifugal switch 70 and contact 124 to speed increasing relay 116, and thence via contact 123 to the positive pole, and brush 43 is similarly connected via centrifugal switch 71 and contact 129 to speed reducing relay 117 and then via contact 123 to the positive pole. The connections and operation of the elements associated with shaft 20-2 are similar to those described for shaft 20-1. Assuming the centrifugal switches are all closed, if the rotor of commutator switch S1 rotates relative to that of switch MS, brush 41 will come into contact with brush 40 and relay 115 will operate which closes contacts 123. Relay 115 is made slow to release by means of a shunting condenser. Either brush 42 or 43 will next come into contact with brush 40 depending on whether the speed of shaft 20-1 is less or greater than that of shaft 20M, so that provided relay 115 is still operated, either relay 116 or 117 will then operate. Relay 116 controls contacts 127 which when closed, establishes a circuit through the field winding 93 of motor 27 which operates a governing mechanism 26 to increase the speed of the engine 22 which drives the shaft 20-1. Relay 117 controls contact 132 which establishes a circuit through field winding 94, causing the motor to run in the reverse direction, thereby reducing the speed of shaft 20-1. The control action impulse lasts until relay 115 releases. The normal delay period of relay 115 is reduced when relay 116 or 117 operates, since these relays, when operated, connect a resistance across the condenser across relay 115. The centrifugal switches 70A and 71A are rotated by the master shaft, and both of these switch contacts are closed while the absolute speed of the master shaft is within a predetermined range ; if the speed is above or below this range one or other switch opens and disables the control system. Switches 70 and 71 are similarly rotated by the shaft 20-1. If the absolute speed of shaft 20-1 is above a predetermined limit, contacts 70 open and disconnect the circuit of the speed increasing relay 116, so that the control system can only act to decrease the speed of the shaft. If the speed of the shaft is below a predetermined limit, the speed reducing relay 117 is similarly disconnected, so that the control action then can only be such as to increase the speed. Auxiliary features includes means whereby relay 117 is rendered inoperable when relay 116 operates and vice versa, and limit switches associated with motor 27 operate at the limits of its travel to light indicating lamps.